In default Netgear firmware scheduling feature is present and that is most probably based on IP address.
I guess your child access internet from a fixed machine. In that case you assign one IP address to that machine and then use scheduling based on that particular IP address. Hope that will fulfill your purpose.

it's simple, enter mac., but read and understand carefully the page that you are stting.in tomato and ddwrt,
time that you set is time where the command will block at that time!
for example: i want to disallow intnet-connection fo ae:45:.....
don't forget to check deny (ddwrt), disallow in-connection(tomnato). set the time from 1.00 pm - 9.00pm , apply change (ddwrt)or save(tomnato). the internet only block only 1 pm until 9pm.

DD-WRT supports MAC based scheduling, see: http://dd-wrt.com/demo/Filters.asp

If you have a WGR614L than you be able to get dd-wrt installed.